Method of determining the strain concentration in rigid articles



Patented July 27, 1943 METHOD OF DETERMINING THE STRAIN CONCENTRATION IN RIGID ARTICLES Greer Ellis, Cambridge, Mass., assignor to Magnaflux Corporation, Chicago, 111., a corporation of Delaware No Drawing. Application August 3, 1940, Serial No. 351,221

1 Claim.

This invention relates to a film forming composition adapted for use in the Method of determining strain concentration in rigid articles disclosed in my application for patent, Serial No. 239,338, filed November 7, 1938 (which has now issued as United States Patent No. 2,294,897) of which the present application is a continua.- tion-in-part. The invention disclosed in said filed application pertains to a method of determining the strain concentration in rigid articles, comprising coating a rigid article with a continuous adherent uniformly brittle film and subjecting said article to increasing loads, when the coating film will crack initially over the most highly strained area of the coated rigid article and subsequently over areas of lower strain concentration, as the load is increased. This phase of the present invention constitutes an improvement of the Film forming composition" disclosed in my application for patent, Serial No. 277,448, filed June 5, 1939, of which the present application is also a continuation-in-part.

A specific improvement over the Film forming composition of the latter filed application disclosed in the present application comprises film forming compositions adapted to coat rigid articles with films clouded by enormous numbers of minute bubbles.

Another specific improvement over said Film forming composition of said filed application comprises a method of selecting, for application under any given conditions of temperature and humidity, film forming compositions operative under said conditions for the determination of strain concentration of rigid articles. The present invention further relates to methods of applying film forming compositions disclosed in the present application.

The exact manner of testing coated rigid articles by subjecting the same to suitable loads is disclosed in my application, Serial No. 239,338, which also describes the various crack patterns formed as a result of such loading.

It is an object of the present invention to provide a film forming composition adapted for use in the direct determination of the location, direction and magnitude of local strain concentration in rigid articles of relatively high elastic limit.

A more Specific object of this invention is to provide compositions of the nature indicated capable of yielding a coating film distinguished by uniform brittleness unafiected by local variations in thickness.

Another specific object of this invention is to provide compositions of the nature indicated capable of yielding brittle coating films which not only respond sensitively by cracking to the impositions of loads on an article coated therewith, but whose cracking also offers an accurate means of measuring the strain concentration in various parts of said coated article.

Further objects of the invention are to provide film forming compositions clouded by an enormous number of minute bubbles as well as to provide methods of applying the compositions of this invention and of selecting from the film forming compositions disclosed those operative under any given conditions of temperature and humidity for application under such conditions.

Other and further objects of this invention will become apparent to those skilled in the art from the following description and appended claim.

I have discovered that a film forming composition comprising a solution of a. brittle natural or synthetic resin in a solvent capable of evaporating quickly and substantially completely from said solution is quite satisfactory for the purposes of said Method of determining strain concentration in rigid articles when plasticized to an extent just sufficient to prevent crazing during drying under the conditions of temperature and humidity which are to prevail during testing. Such compositions yield films distinguished by a uniform brittleness independent of local variations in thickness, and by a capacity for responding by cracking quickly, sensitively and accurately to variations in strain concentration in rigid articles coated therewith.

I have further discovered that film formin compositions according to this invention, plasticized to render the same operative under given conditions of temperature and humidity, are also operative under certain other and difierent conditions of temperature and humidity. Further,

all the sets of conditions of temperature and humidity under which a specific film forming composition is operative are so related to each other and to other sets of temperature and humidity conditions under which other specific film forming compositions are operative that charts in which humidity is plotted against temperature may be constructed, showing contiguous areas representing operative conditions for specific compositions. By means of such charts the specific composition operative in any given set of temperature and humidity conditions may be selected. p

I have also discovered that compositions comprising the minimum or near-minimum amounts ditions the films will craze spontaneo of solvents necessary to yield a smooth film, when sprayed with the maximum or near-maximum amounts of air effecting the formation of smooth films, form films clouded with enormous numbers of bubbles too minute to be discernible to the naked eye. Such bubble-clouded films appear milky and, while no more brittle and hence no more sensitively responsive by cracking to straining than clear films, yet are far more accurately responsive to straining. One possible explanation for this fact is that the bubbles subdivide the films into a large number of minute sections which all have the same cracking characteristics.

l br preparing film-forming compositions according to the present invention, light colored hardened natural resins are preferable, although hard brittle synthetic resins may also be used.

Among the natural resins may be mentioned gum resin, wood resin, dammar and various copals such as Manila, Kauri, Congo and the like. Hardening of the natural resin may be effected by treatment with lime, either by fusion or in the presence of water, by treatment with other calcium compounds, such as calcium acetate, by treatment with basic hydroxides or oxides such as magnesia or zinc oxide, by oxidation with a current of hot air, and in other manners known to the art relating to natural resins. The hardened resins may thus comprise a metallic resinate or oxidation products of a natural resin, or both. Various types of lime-hardened resins, especially wood resin treated with large amounts of lime or calcium acetate, or both, have been found particularly useful. Reaction products of a resin or rosin with zinc oxide are also well suited for the purposes of this invention.

The plasticizer used should have a very low evaporation rate, to insure that the film, once formed, should retain its mechanical properties unchanged for considerable periods of time. It should also be compatible with the resin and solvent used, both in the film forming composition and in the film. Numerous substances of the type commonly used as plasticizers have been found useful as plasticizers for the purposes of the present invention. Dibutyl phthalate and normal butyl stearate may be mentioned in particular. The former is preferably used in conjunction with carbon disulfide; the latter, in conjunction with methylene dichloride.

Solvents should be capable of dissolving the resin and plasticizer at all stages during the formation of the film. They should not have a tendency to absorb moisture from the air, should evaporate rapidly without leaving a residue, and no appreciable parts or fractions thereof should be retained by the dried film, In general, "Nonsolvating solvents, or those solvents which do not tend to be absorbed by or to form loose compounds or complexes with the dissolved resin, are those desired for the purposes of the present invention. Low boiling solvents are desirable, to efiect rapid evaporation and drying. A pure compound is preferable, since it will evaporate uniformly, without leaving behind slow drying higher boiling fractions. Non-polar, low-boiling liquid compounds of carbon are particularly suitable. Carbon disulfide, methylene dichloride and 1,2 (cis) dichlor ethylene are examples of such liquid carbon compounds.

Brittle films formed by such compositions are operative only at certain temperatures. At certain minimum temperatures and humidity conthe right represent higher temperatures and humidities the films will lose some of their brittleness, hence also some of their sensitiveness in responding by cracking to straining, tending to fiow plastically rather than to crack.

A series of ten compositions of graduated brittleness operative at any combination of a temperature ranging from 60 to F. and with a humidity of from 0 to 90 per cent relative humidity may be prepared by adding to ten solutions of one part by weight of an extremely brittle resin, such as a highly limed wood rosin, in two parts of a volatile unitary solvent, such as carbon disulfide, a plasticizer, for instance, dibutyl phthalate, in the following amounts: 0.04, 0.045, 0.05, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.085, parts by weight. The amounts of plasticizer may have to be varied somewhat to compensate for variations in brittleness as between various batches of resin.

The humidities and temperatures at which each of these ten compositions craze spontane- .ture and moisture conditions causing each such lacquer to be inoperative due to crazing. The

areas to the right of each line, all the way to the adjacent line to the right, represent tempera. ture and humidity conditions under which each composition is operative, while those further to conditions under which sensitivity and accuracy are lessened due to loss 01' brittleness of the films. The correct composition for use under any given conditions of temperature and humidity may easily be selected by consulting such a chart.

For the preparation of compositions forming films clouded with bubbles, from 90 to 140 parts, by weight, of solvent may be used for each 50 parts of resin, although the preferred range is from to parts. From 2 to -5 parts of plasticizer may -be used, depending on the exact brittleness desired.

Example 50 parts of a resin-like product having a melting point (capillary tube) between and C. and comprising the reaction product of rosin and at least 6 per cent by weight of zinc oxide are dissolved in 107 parts'by, weight of carbon disulfide and about 3'parts of dibutyl phthalate are added.

Compositions such as the composition described in the preceding paragraph as an illustrative example may be applied by dipping, brushing, or, preferably, by sprayin Only the latter method will cause clouding by bubbles. Films from 0.003 to 0.008 inch in thickness are obtained after at least 6 hours drying, distinguished by uniform brittleness independent of local variations in thickness. During drying and testing the temperature should not vary more than 5 F: The humidity ordinarily remains practically constant.

It will be understood that numerous details oi. composition and application may b varied within a wide range without departing from the principles of this invention and it is therefore not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claim.

I claim as my invention:

The method of determining the relative distriareas of a rigid article of relatively high elastic limit which comprises applying to said areas a film-forming composition including a brittle resin plasticized barely sufiiciently to prevent crazing under testing conditions and drying by evaporation to form a uniformly brittle film capable of crack formation under the influence of gradually increased loads, said composition being applied in amounts and in a manner such a after drying of said composition to coat said areas with a brittle film having a uniform thickness of from 10 0.003 to 0.008 inch, drying said composition to effect the formation of brittle film having a thickness of from 0.003 to 0.008 inch, and subjecting 

